Surface Analysis of Magnesium-Based Intermetallic Compounds

Abstract

Introduction Magnesium-based rechargeable battery is expected as a candidate of beyond lithium-ion battery1. Since the magnesium metal anode does not form dendrite during the deposition process, most of the studies for the magnesium battery are based upon the magnesium metal anode. Typically the electrochemical deposition/dissolution process of the magnesium metal has never been observed in conventional ionic electrolytes such as Mg(ClO4)2 or Mg(PF6)2. Recently it was reported that Mg(TFSI)2 in glyme-based solvents showed some reversibility in Mg deposition/dissolution2, 3. However the overpotential of the Mg deposition/dissolution process in the Mg(TFSI)2-based electrolyte solution is relatively high > 1.0 V. Thus it is still difficult to utilize magnesium metal as an anode active material for a rechargeable magnesium battery with a conventional ionic electrolyte. We have been working on magnesium-based intermetallic compounds as alternative anode active materials for magnesium batteries, because the intermetallic compounds are compatible with conventional ionic electrolytes4. Here we conducted electrochemical and surface analyses of Mg3Bi2 thin film electrodes in various electrolyte solutions to understand the electrochemical activity of the magnesium-based intermetallic compounds. Experimental The Mg3Bi2 thin film electrodes were fabricated co-sputtering process of magnesium and bismuth on copper substrate at 200 ºC. A three-electrode cell with Mg3Bi2 thin film working electrode was fabricated in an Ar filled glove box. A double junction Ag/Ag+ reference electrode and a platinum counter electrode were employed for the electrochemical measurement. Surface analyses were carried out using in situ FTIR based upon a diamond ATR optical system, and X-ray photoelectron spectroscopy (XPS). Results The obtained thin film electrode was characterized using XRD. A highly crystallined Mg3Bi2 thin film with small amount of Bi was obtained via the RF-sputtering without any post-annealing process. It suggests that the substrate temperature 200 ºC is high enough to form the Mg3Bi2 thin film. A cyclic voltammogram for the Mg3Bi2 thin film electrode in 0.5 M Mg(TFSI)2 BuMe-triglyme solution, is shown in Figure 1. Very clear cathodic and anodic currents were observed at -2.3 V vs. Ag/Ag+. The overpotential was approx. 100 mV while the magnesium deposition/dissolution process in the particular electrode shows extremely high overpotential approx. 2.0 V. Thus we think the Mg3Bi2 intermetallic compound has clear advantage against the magnesium metal in terms of the energy efficiency. The XPS analysis results suggested that the TFSI anion was reduced at the surface of the Mg3Bi2 thin film electrode during the electrochemical measurement, however the amount of the decomposed products were much less than that on magnesium metal. Further discussion will be done in the session.